The invention concerns a receiver circuit, in particular a receiver circuit for audio frequency modems, comprising means for receiving and sampling an analog input signal representing digital data, an adaptive equalizer having a transversal filter with a plurality of complex multiplier coefficients ordered relative to a usually central highest norm coefficient to generate symbols at a timing rate from a sampled analog signal and a timing recovery device.
The document IEEE TRANSACTIONS ON COMMUNICATIONS, Vol.comm-26, No 5, "Passband Timing Recovery in All-Digital Modem Receiver--D. Godard", dated 5 May 1978, discloses a timing recovery device for a quadrature amplitude modulation modem in which a timing signal is recovered from two characteristic frequencies in the spectrum of the received signal on opposite sides of the frequency band used to transmit the digital data. For a modulated 1800 Hz carrier type received signal the two characteristic frequencies are 600 Hz and 3000 Hz. The basic principle of timing recovery is as follows. If at a given time there exists within the spectrum of the received signal a signal at a frequency near 600 Hz (604 Hz, for example) there is also a signal at a frequency near 3000 Hz (3004 Hz, for example). This property enables calculation of representative values of timing drift used to control an oscillator. Two filters respectively centered on the frequencies of 600 Hz and 3000 Hz are used for this. The filter centered on 600 Hz outputs sin(604 Hz) and cos(604 Hz) signals. The filter centered on 3000 Hz outputs sin(3004 Hz) and cos(3004 Hz) signals. The following trigonometrical equation applies: EQU sin (2400 Hz)=sin (3004 Hz). cos (604 Hz)-cos (3004 Hz). sin (604 Hz)
The value of the signal sin(2400 Hz) sampled at 2400 Hz is used to control the oscillator supplying the timing signal.
A timing recovery device of this kind has drawbacks and in particular it cannot be used for transmission lines which attenuate the analog signal near the characteristic frequency of 3000 Hz. Also, it is standard practice to use the sidebands available in the frequency spectrum of the analog signal to transmit service data. The existence of the two characteristic frequencies used for timing recovery makes the use of the sidebands more difficult.